Study On The Improvement Of Breakdown Strength Of TiO₂-based Ceramic Dielectric Materials By Ca₉Al（PO₄）₇

Pulsed power technology has important applications in the military,medical and environmental protection fields.Pulse formation line is the core component of the pulse power system,not only to withstand the high voltage and the high power energy released by the upper level of temporary storage,but also to play the role of compression pulse width.In the traditional high-power pulse system,the liquid dielectric material is often used as the pulse formation line energy storage medium,but its environmental adaptability,energy storage density is difficult to further enhance,which contradict with the development of the pulse power device miniaturization.Ceramic medium material is an excellent alternative material due to its superior dielectric properties and good environmental adaptability.However,its breakdown strength cannot meet the development needs of pulsed power systems,hence,improving the breakdown strength of ceramic medium has become the focus of research.In this dissertation,TiO₂-SiO₂-Al₂O₃-based dielectric ceramics were selected as the object of study,and Ca₉Al（PO₄）₇ was introduced as the second crystalline phase by in-situ synthesis and pre-synthesis,respectively.The study investigated the second crystalline phase effects on the microstructure,breakdown strength and dielectric properties of the ceramics,the results as follows:（1）TiO₂-SiO₂-Al₂O₃-x（x=0～1.65）Ca₃（PO₄）₂ ceramics were prepared by solid phase method,and the second crystalline phase Ca₉Al（PO₄）₇ were introduced by in-situ synthesis method.The content of Ca₉Al（PO₄）₇ was tuned by changing the composition of raw materials.The SEM images showed that Ca₉Al（PO₄）₇ crystalline phase appeared between the TiO₂grains.With the increase of x,the breakdown strength increased first,and then decreased.The breakdown strength of TiO₂-based ceramics was 90.8 k V/mm when x=1.32,the largest of all,which was 1.83 times higher than the breakdown strength of x=0.And it has good dielectric properties.The high-temperature impedance spectrum showed that the second crystalline phase Ca₉Al（PO₄）₇ increased the grain boundary and grain conductivity activation energy,which implies an increase in the energy potential barrier,making the grains and grain boundaries more capable of trapping electrons,thus increasing the breakdown strength.（2）TiO₂-based glass composite ceramics were prepared by introducing the Pre-synthesized Ca₉Al（PO₄）₇ glass powders into TiO₂.The crystalline phase composition,microscopic morphology and breakdown strength of TiO₂-based glass composite ceramics were improved by changing the Ca₉Al（PO₄）₇ glass content.With the increase of Ca₉Al（PO₄）₇glass addition,the second crystalline phase Ca₉Al（PO₄）₇ content in the ceramics increased,and the grain size gradually decreased and became uniform.The breakdown strength increases from 48.9 k V/mm to 91.4 k V/mm.And it has good dielectric properties.The high-temperature impedance spectrum showed that,the second crystalline phase Ca₉Al（PO₄）₇was beneficial to improve the energy potential barrier;increase of its content and decrease of its grain size leaded to an increase in grain boundary density,which increased the number of trapped free charge sites.The ceramics breakdown strength was increased by the combined effect of both.